Melt-spinning (or spunbonding) is the process of forming fibers by extruding molten polymer through small orifices in a die, collecting the spun filaments on a belt in a uniform random fashion, and bonding the fibers to form a cohesive web. Melt-blowing (or MB) is the process of forming fibers by extruding molten polymer through small orifices surrounded by high speed heated gas jets, and collecting the blown filaments as a cohesive web. This process is also referred to as a blown micro fiber (or BMF) process. The most common thermoplastic material used for the BMF process is polypropylene, which produces a very fine fiber with good thermal stability.
Polyesters such as poly(ethylene)terephthalate (PET) and polyolefins such as poly(propylene) (PP) are two commonly used classes of petroleum based polymers in the commercial production of textile fibers, packaging films, beverage bottles, and injection molded goods by processes such as BMF and spunbond. Although PET has a higher melting point and superior mechanical and physical properties compared to other commercially useful polymers, it exhibits poor dimensional stability at temperatures above its glass transition temperature. Polyester fibers, e.g. aromatic polyesters such as PET and poly(ethylene)terephthalate glycol (PETG), and/or aliphatic polyesters such as poly(lactic acid) (PLA), and webs including such fibers, may shrink up to 40% of the original length when subjected to elevated temperatures due to the relaxation of the oriented amorphous segments of the molecules to relax upon exposure to heat (See Narayanan, V.; Bhat, G. S. and L. C. Wadsworth. TAPPI Proceedings: Nonwovens Conference & Trade Fair. (1998) 29-36).
Furthermore, PET has generally not been considered as suitable for applications involving high-speed processing because of its slow crystallization from the melt state; at commercial production rates, the polymer has minimal opportunity to form well developed crystallites. Articles prepared from PET fibers typically need to undergo an additional stage of drawing and heat-setting (e.g. annealing) during the fiber spinning process to dimensionally stabilize the produced structure.
Additionally, there is also a growing interest in replacing petroleum-based polymers, such as PET and polypropylene (PP), with resource renewable polymers, i.e., polymers derived from plant based materials. Ideal resource renewable polymers are “carbon dioxide neutral” meaning that as much carbon dioxide is consumed in growing the plants base material as is given off when the product is made and disposed of. Biodegradable materials have adequate properties to permit them to break down when exposed to conditions which lead to composting. Examples of materials thought to be biodegradable include aliphatic polyesters such as poly(lactic acid) (PLA), poly(glycolic acid), poly(caprolactone), copolymers of lactide and glycolide, poly(ethylene succinate), and combinations thereof.
However, difficulty is often encountered in the use of aliphatic polyesters such as poly(lactic acid) for BMF due to aliphatic polyester thermoplastics having relatively high melt viscosities which yields nonwoven webs that generally cannot be made at the same fiber diameters that polypropylene can on standard nonwoven production equipment such as spunbond and BMF production lines. The coarser fiber diameters of polyester webs can limit their application as many final product properties are controlled by fiber diameter. For example, course fibers lead to a noticeably stiffer and less appealing feel for skin contact applications. Furthermore, course fibers produce webs with larger porosity that can lead to webs that have less of a barrier property, e.g. less repellency to aqueous fluids.
The processing of aliphatic polyesters as microfibers has been described in U.S. Pat. No. 6,645,618 (Hobbs et al.) and U.S. Pat. No. 6,645,618. U.S. Pat. No. 6,111,160 (Gruber et. al.) discloses the use of melt stable polylactides to form nonwoven articles via melt blown and spunbound processes. JP6466943A (Shigemitsu et al.) describes a low shrinkage-characteristic polyester system and its manufacture approach. U.S. Patent Application Publication No. 2008/0160861 (Berrigan et al.) describes a method for making a bonded nonwoven fibrous web comprising extruding melt blown fibers of a polyethylene terephthalate and polylactic acid, collecting the melt blown fibers as an initial nonwoven fibrous web, and annealing the initial nonwoven fibrous web with a controlled heating and cooling operation. U.S. Pat. No. 5,364,694 (Okada et al.) describes a polyethylene terephthalate (PET) based meltblown nonwoven fabric and its manufacture. U.S. Pat. No. 5,753,736 (Bhat et al.) describes the manufacture of polyethylene terephthalate fiber with reduced shrinkage through the use of nucleation agent, reinforcer and a combination of both.
U.S. Pat. Nos. 5,585,056 and 6,005,019 describe a surgical article comprising absorbable polymer fibers and a plasticizer containing stearic acid and its salts.
Thermoplastic polymers are widely employed to create a variety of products, including blown and cast films, extruded sheets, foams, fibers, monofilament and multifilament yarns, and products made therefrom, woven and knitted fabrics, and nonwoven fibrous webs. Traditionally, many of these articles have been made from petroleum-based thermoplastics such as polyolefins.
Degradation of aliphatic polyesters can occur through multiple mechanisms including hydrolysis, transesterification, chain scission, and the like. Instability of such polymers during processing can occur at elevated temperatures as described in WO94/07941 (Gruber et al.).
Many thermoplastic polymers used in these products, such as polyhydroxyalkanoates (PHA), are inherently hydrophobic. That is, as a woven, knit, or nonwoven such as a spunbond fabric, they will not absorb water. There are a number of uses for thermoplastic polymers where their hydrophobic nature either limits their use or requires some effort to modify the surface of the shaped articles made therefrom. For example, polylactic acid has been reported to be used in the manufacture of nonwoven webs that are employed in the construction of absorbent articles such as diapers, feminine care products, and personal incontinence products (U.S. Pat. No. 5,910,368). These materials were rendered hydrophilic through the use of a post treatment topical application of a silicone copolyol surfactant. Such surfactants are not thermally stable and can break down in an extruder to yield formaldehyde.
U.S. Pat. No. 7,623,339 discloses a polyolefin resin rendered antimicrobial and hydrophilic using a combination of fatty acid monoglycerides and enhancer(s).
Coating methods to provide a hydrophilic surface are known, but also have some limitations. First of all, the extra step required in coating preparation is expensive and time consuming. Many of the solvents used for coating are flammable liquids or have exposure limits that require special production facilities. The quantity of surfactant can also be limited by the solubility of the surfactant in the coating solvent and the thickness of the coating.
Post treatment of the thermoplastic polymer can be undesirable for at least two other reasons. First, it can be more expensive since it requires additional processing steps of surfactant application and drying. Second, PHAs are polyesters, and thus prone to hydrolysis. It is desirable to limit the exposure of PHA polymers to water which can be present in the surfactant application solution. Furthermore, the subsequent drying step at elevated temperature in the wet web is highly undesirable.